Inmarsat L-TAC

Beyond Line of Sight Communications Using Existing Military Radios

The military in general and troops in the land environment in particular rely on robust, portable radio terminals to provide all-informed communications for command, control and coordination of dispersed force elements. However, when operational distances extend beyond line of sight, the normal workhorse VHF terminals run out of range and are unable to meet the capability requirement without the use of rebroadcast (or relay) stations. These lead to additional force protection and force sustainment requirements which, in turn, create logistic and combat tasks that potentially divert assets from the main effort.

The alternative is Single Channel (SC) tactical satellite (TACSAT), conventionally provided in the UHF band on military satellites. The demand for these channels exceeds supply, meaning that nations are often unable to lease or gain access to channels. Alternatively, due to the limited availability of channels, they are forced to operate user groups that are far larger than the optimal size. These large groups result in the need for restrictions on the use of the channel. If these are not strictly observed, net discipline can collapse quickly, causing confusion and dragging down operational tempo.

The Challenge
To command widely dispersed forces in challenging environments, without the delay of deploying terrestrial infrastructure where UHF TACSAT channels are both expensive and in short-supply.

The Requirement
Increase the number of TACSAT channels for voice and data over tactical, theatre and strategic distances using existing tactical radios with their in-built encryption.

The Solution
SlingShot ® - Spectra’s small, Communications on the Move (COTM) system.


L-TAC - Inmarsat’s global constellation of geostationary satellites.


Inmarsat’s simple but innovative approach to making more TACSAT channels available to the market exploits the ability of the Inmarsat-4 (I-4) satellites to connect L-band transponders on the spacecraft to provide the same single-hop relay capability in space as is provided by military UHF.

Channels are engineered to offer a 25kHz channel for both Voice and / or Low Speed Data (LSD), equal to that provided by UHF. The 25kHz channels can be divided into up to five 5kHz sub-channels each providing Narrow Band Voice and/or LSD using ANDVT mode.ange this text.

The L-TAC service is available throughout the the global footprint of the I-4 constellation. Whilst individual regional and narrow footprint than the UHF equivalent, the sophisticated capabilities of the Inmarsat network allow one beam when required. Customised covering the user’s area of operations will be the resources being available on the constellation when ordered.

The service is offered on the I-4 satellites, which are currently located at 98°W, 25°E and 143.5°E and provide global coverage. L-band coverage is offered over the standard narrow (1.4° diameter) and regional (4.6° diameter) beams of the I-4 satellites.

Example Narrow Beam
Example Regional Beam


Slingshot Overview


  • Radio Agnostic for UHF and VHF systems
  • Sustained communications on the move
  • No need to deploy antenna
  • Manpack Vehicular/Maritime and Ops Room Options

To avoid the need for large-scale capital investment in developing and buying new fleets of tactical radios compatible with the L-TAC service, Inmarsat’s partner, Spectra, has developed the SlingShot range of products for use on the Inmarsat network.

The converter unit’s design allows it to be operated remotely e.g. from inside a building, with the L-band antenna separated from the appliqué by up to 2 metres and the appliqué by up to a further 10 metres from the UHF radio. The ability to site the antenna remotely from the radio also allows the operator to achieve line of sight from antenna to satellite while he himself remains concealed from view and from fire.
The initial product release features omni-directional antennas for Comms On The Move (COTM) applications. A directional antenna will be available from 2014 and will allow a high quality link using lower power when operating at the pause (COTP) or from static locations. It will also allow the option of leasing full-channel (25 kHz) wideband data in an I-4 regional beam.

Spectra’s compact [170 mm x 87 mm x 29 mm; 500g] vehicle unit frequency converter (appliqué) is engineered and certified to military standards for shock protection, EMC and ingress prevention. It is powered from a separate intelligent power supply that draws power from a 9V – 36V source and includes a self-diagnostic capability display provided on four LEDs. The unit accepts an RF input between 281 MHz and 311 MHz from a host radio, which it converts to L-band for transmission to the satellite and carries out the reverse conversion for the returned signal, providing a half-duplex channel equivalent to SC TACSAT. It is capable of supporting most military TACSAT radio types.